1. Field of the Invention
This invention relates to a sintered alloy, e.g., for cutting tools, with advantageous properties.
2. Description of the Prior Art
WC-type hard metals and TiC-type cermets are available as sintered alloys for cutting tools which are used for turning, milling, etc. WC-type hard metals have superior thermal conduction and high toughness, but because of great crater wear, suffer from the defect that the edge of a cutting tool made of it tends to be damaged. Accordingly, the application of these metals to high speed cutting in current use is limited within a narrow range.
On the other hand, TiC-type cermets are suitable for high speed cutting because of their high hardness and superior thermal resistance. However, since they have low toughness and thermal shock resistance and tend to undergo plastic deformation at high temperatures, they suffer from the defect that chipping occurs in milling, intermittent cutting, etc., and when heavy cutting is performed, or when the workpiece has a high hardness, the cutting edge will be deformed and the life of the tool shortened.
Both of these types of alloys have therefore had only limited applications even in view of their advantages because of their disadvantages at the same time.
Attempts have been made to increase the toughness of TiC-type cermets by replacing not more than 50% by weight of the TiC as their main ingredient by another carbide such as WC, TaC or HfC. But these modifications only have made possible intermittent cutting of a small degree, and satisfactory alloys cannot be obtained.
TiC-type cermets resulting from the replacement of a part of the TiC by a metal nitride such as TiN or ZrN are also known. In this case, the cermets have improved shock resistance, and are effective in milling, and wet-type cutting involving thermal shock. However, since such a metal nitride has low hardness and tends to cause plastic deformation, such cermets have been directed to limited applications in which the cutting edge is maintained at low temperatures.
In the present invention a sintered alloy has been developed which comprises a TiC-type cermet for cutting tools, which obviates the defects of the WC-type hard metals and TiC-type cermets. Such can be applied to a wide range of use, particularly has enhanced resistance to plastic deformation without reducing the superior crater resistance of TiC-type cermets thus making possible heavy cutting at high speeds, further has improved thermal conduction and higher thermal shock resistance and mechanical shock resistance than TiC-type cermets resulting from replacing a part of TiC by another carbide or nitride, and is suitable for milling and wet-type cutting at high speeds.
A TiC-type cermet is composed basically of TiC, Ni and Mo. Mo, used conjointly with Mo.sub.2 C or replaced by Mo.sub.2 C diffuses into the surface of TiC due to its very high wettability thereby increasing the bond strength between the TiC and Ni. When the amount of Mo is excessive, a brittle intermediate phase [(Ti, Mo)C] is formed reducing the strength of the alloy. It has therefore been believed that the amount of Mo and/or Mo.sub.2 C must be limited to not more than 20% by weight.
Repeated experiments on TiC-type cermets using various metal carbides, have now been conducted and consequently it has been found that by adding Ni and Co as binder metals to a ternary composition of TiC-WC-Mo.sub.2 C in proportions shown by the range surrounded by A, B, C and D in FIG. 1, and forming and sintering the mixture, an alloy can be obtained which has markedly improved resistance to plastic deformation, enhanced thermal conduction and greatly improved thermal shock resistance. However, grain growth tends to occur in this alloy, and therefore the alloy has low mechanical shock resistance and unsatisfactory abrasion resistance.
In order to inhibit the grain growth which is the defect of the above alloy, repeated experiments were conducted, and satisfactory results have finally been obtained by incorporating 5 to 20% by weight of a nitride selected from TiN and/or TaN together with 5 to 20% by weight of a binder composed of Ni and/or Co.